What memcached does that Redis doesn't do is least-recently-used eviction of values from the cache. With memcached, you can safely set as many values as you like, and when they overflow memory, the ones you haven't used recently will be deleted. With Redis, you can only approximate this, by setting a timeout on everything; when it needs to free up memory, it will look at three random keys and delete the one that's the closest to expiring.
That's the main difference, if you're just using it as a cache.
